1. Field of the Invention
The present invention is related to a hydraulic control system for a vehicular continuously variable transmission and, more particularly to a hydraulic control system for controlling a forward and reverse changeover apparatus and a lockup clutch incorporated in the continuously variable transmission.
2. Discussion of the Prior Arts
There is known a belt-and-pulley type continuously variable transmission whose speed ratio is continuously variable. The continuously variable transmission comprises a power transmission mechanism serving to change speeds, a torque converter serving as a starting device for assisting the start of the vehicle, a forward and reverse changeover apparatus serving to change the running directions, forward and reverse, of the vehicle, a lockup clutch provided in the torque converter for mechanically engaging the crankshaft with the power transmission mechanism and a hydraulic control apparatus for operating or controlling the transmission mechanism, the forward and reverse changeover apparatus, the lockup clutch and others.
The power transmission mechanism includes an input and an output shaft, a pair of variable-diameter pulleys provided on the input and output shafts, respectively, and a belt connecting the pulleys for transmitting power therebetween. The speed ratio between the input and output shafts is obtained by changing the effective diameter of the pulleys through hydraulic pressure generated in the hydraulic control apparatus.
The forward and reverse changeover apparatus includes a planetary gear assembly, a forward clutch and a reverse brake. When a driver operates a select lever to select a forward range, the forward clutch is engaged by the help of a hydraulic pressure supplied from the hydraulic control apparatus. Further, when a reverse range is selected, the reverse brake is applied.
The lockup clutch is also controlled by the hydraulic control apparatus and when the vehicle speed exceeds a specified value the hydraulic pressure is applied to an actuator of the lockup clutch so as to connect an impeller with a turbine of the torque converter.
Japanese Patent Application Laid-open No. Toku-Kai-Shou 57-161360 discloses a hydraulic control system in which the diameter of the pulleys, the engagement or disengagement of the forward clutch and the application or releasing of the reverse brake and the engagement or disengagement of the lockup clutch are controlled through a line pressure supplied from one unit of an oil pump driven by an engine. This type of hydraulic control system has a defect in that the variation of the line pressure caused by the fluctuation of engine torque or accompanied by the speed ratio control may affect the operation of the forward clutch and the like adversely.
As a technique for solving the problem, Japanese Patent Application Laid-open No. Toku-Kai-Shou 63-297852 proposes a hydraulic control system having two oil pumps, one for high pressure and the other for low pressure in which the speed ratio control is performed by the high pressure oil pump and other controls such as the forward clutch control and the lockup clutch control are dependent on the low pressure oil pump.
As described in Japanese Patent Application Laid-open No. Toku-Kai-Hei 3-125074, there is a forward and reverse changeover apparatus including a double-pinion type planetary gear assembly and friction engagement elements. Since this type of forward and reverse changeover apparatus is designed such that the torque exerted on the reverse brake when the reverse range is selected is double that exerted on the forward clutch when the forward range is selected, the reverse brake must be large in size compared to the forward clutch because of its increased friction force capacity.
On the other hand, in a case where a driver moves the select lever to the reverse range inadvertently during running forwardly, the reverse brake is engaged before the forward clutch is released and an "interlocking" phenomenon occurs between the forward clutch and the reverse brake, this incurring an abnormal shock or leading to a burning in the reverse brake. To prevent the "interlocking", there is a known technique that no restrictor is provided in the drain passage from the forward clutch in order to discharge working fluid rapidly and a restrictor is provided in the oil supplying passage to the reverse brake so as to delay the engagement of the reverse brake.
However, this restrictor has an adverse effect in that when the select lever is placed from the neutral (N) position to the reverse (R) position under the state of idling, a driver may feel some degree of delay in the vehicle starting due to the delay of the engagement of the reverse brake.
With respect to a technique to prevent the inadvertent shift to the reverse (R) range, Japanese Patent Application Laid-open No. Toku-Kai-Hei 4-357357 proposes a hydraulic control system having a safety-lock valve to release hydraulic pressure from the oil chamber of cylinders for operating the forward clutch and the reverse brake, respectively.